ci: validate commit messages instead of PR title

[Christopher Co (christopherco)]

We rebase-merge, so every commit enters the permanent 4.0 history. The
old check only validated the PR title, which could hide non-conventional
commits behind a conventional title (or fail a well-formed branch behind a
descriptive one).

Validate the header of every commit instead. A github-script step reads
the PR commits via the API and checks each against the Conventional Commit
types in CONTRIBUTING.md, allowing optional scopes and breaking-change
markers. On failure it comments the offending commits and how to fix them,
and removes the comment once all are valid.

The workflow stays on pull_request_target so it can comment on fork PRs,
but only reads commit metadata through the API and never checks out or
runs PR code. Commit subjects are HTML-encoded before display, the count
is checked against the PR total to fail closed on the 250-commit API cap,
and all actions remain SHA-pinned.

Fixes: AB#22437

[Copilot]

Pull request overview

Replaces PR-title linting with per-commit Conventional Commit validation for rebase-merged 4.0 history.

Changes:

• Validates all PR commit headers through the GitHub API.
• Posts sanitized diagnostics and remediation guidance.
• Clarifies commit and PR-title requirements.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
CONTRIBUTING.md	Documents per-commit CI validation.
.github/workflows/check-pr-title.yml	Removes PR-title validation.
.github/workflows/check-commit-messages.yml	Adds commit-message validation and feedback.

---

💡 Add a code-review agent skill or configure MCP servers for context-aware, tailored reviews. Learn more in the docs.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

Suppressed comments (3)

.github/workflows/check-commit-messages.yml:17

• pull_request_target loads workflow definitions only from the repository's default branch, but the default-branch workflow listing still contains check-pr-title.yml and not this new file. As a result, merging this change into 4.0 will neither start this validator nor retire the title validator. Add a default-branch stub for this check (and remove/replace the old default-branch workflow), delegating to a trusted reusable workflow on 4.0, as done by .github/workflows/check-rendered-specs-stub.yml:1-38.

  pull_request_target: # zizmor: ignore[dangerous-triggers]

.github/workflows/check-commit-messages.yml:125

• This builds the comment from every invalid commit, but GitHub rejects issue/PR comments over 65,536 characters. With up to 250 commits and each 200-character subject expanding into HTML entities, list can exceed that limit several times over, so the sticky-comment step fails and users receive none of the promised details. Cap the displayed entries and report how many were omitted.

            const list = invalid
              .map((c) => `- \`${c.sha}\` <code>${encodeSubject(c.subject)}</code>`)
              .join('\n');

.github/workflows/check-commit-messages.yml:166

• A missing output is treated as an empty string by Actions expressions, so this deletion also runs when validate_commits crashes before setting error_message (for example, on an API failure). That removes the previous diagnostic even though validation did not succeed. Require the validation step to have succeeded before deleting the sticky comment.

      - if: steps.validate_commits.outputs.error_message == ''

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

Suppressed comments (3)

.github/workflows/check-commit-messages.yml:59

• A whitespace-only subject such as feat: passes this regex because .+ accepts spaces, even though the documented format requires a non-empty short summary. Require the first summary character to be non-whitespace and anchor the match so the whole header is validated.

              ': .+'                          // ': ' followed by a non-empty summary

.github/workflows/check-commit-messages.yml:94

• This per-subject limit does not bound the final comment size after encoding. With up to 250 invalid commits, disallowed or non-ASCII characters expand into numeric entities, so the generated body can grow to hundreds of kilobytes and exceed GitHub's 65,536-character issue-comment limit; the sticky-comment step then fails instead of reporting the offending commits. Cap the aggregate rendered list (while preserving every SHA and marking truncated subject previews), or choose a per-entry budget derived from the API limit.

              const firstLine = String(text || '').split('\n')[0].slice(0, 200);

.github/workflows/check-commit-messages.yml:141

• Changing the sticky-comment header leaves existing pr-title-lint-error comments behind on open PRs. Those comments continue telling contributors that titles must use Conventional Commits, which directly contradicts the new policy; moreover, title edits no longer trigger this workflow. Add a migration cleanup for the legacy header and ensure an event can run it for existing PRs (for example, temporarily retain edited).

          header: commit-message-lint-error

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

Suppressed comments (2)

.github/workflows/check-commit-messages.yml:171

• The removed workflow used the sticky-comment header pr-title-lint-error, but this replacement only deletes commit-message-lint-error. Existing open PRs with a title-lint comment will therefore retain a bot message claiming their title must follow Conventional Commits, directly contradicting the new policy. Delete the legacy header as part of this migration.

      # Delete the previous comment once every commit message is valid.
      - if: steps.validate_commits.outputs.error_message == ''
        uses: marocchino/sticky-pull-request-comment@5770ad5eb8f42dd2c4f34da00c94c5381e49af88 # v3.0.5
        with:
          header: commit-message-lint-error
          delete: true

.github/workflows/check-commit-messages.yml:60

• The pattern treats whitespace as a non-empty summary, so a commit such as fix: passes even though it has no <short summary> as required by CONTRIBUTING.md. Require the first summary character to be non-whitespace and anchor the header to the end of the subject.

              ': .+'                          // ': ' followed by a non-empty summary